I have replaced my 6 volt with a 12 volt system. I bought all reproduction 12 volt gauges to keep the original look. All my gauges work well but by temperature gives me fits. The gauge has 3 terminals all clearly marked. One for the sender, one for power the other ground. When it is hooked up like this the needle barely moves if at all. When I disconnect the negative wire the gauge seems to work normally but it quickly rises to Hot. I put a laser temp gauge on the heads and they read around 190 -200. Meanwhile the gauge is pegged to the right. Gauge came with them sensor.

First, what is the appropriate head temperature when running normally? Is it really running hot or is the gauge off?
Second is whats up with the gauge itself. Maybe the answer to the first question answers the second question.

Thanks in advance guys.

 

What engine do you have?
Head temps when up to temp in the range you are seeing is pretty common. Sounds like a bad gauge or sender.

 

If it is the replacement gauge and sender from MidFifty the midpoint is set at 160. I had the same type of problem where the gauge went to hot even though everything else told me it wasn't. To make it so I could use the dash gauge instead of a bolt on, I took the gauge to a local place and had them set the midpoint to 210. They added a resistor to it to change it from the 105 ohms that it was set to. I am not smart enough or patient enough to do that. I have a 302 with the 180 thermostat in mine and the truck runs at 201 on a really hot day. I actually just walked in the house from putting it in and taking it for a test run and it works great. I used the permatex sealant to make sure I didn't mess up the contact and to keep it from leaking.

 

I pretty much had the same experience on a recent temp gauge install. If the OP's wiring is correct he may need a resistor in series with his gauge and his temp sender. A lot of gauges register center with the temp sensor reading 100 ohms. So if your gauge reads 3/4 to the right during normal engine temp, add a 50 ohm resistor in series with the gauge and sensor, and should help you get a more centered temp reading. I know it's kind of shoot from the hip, but worth a try. You could use a 100 ohm resistor to test the gauge. Disconnect the gauge from the sensor. Tie the 100 ohm resistor from the disconnected sensor wire to ground. The gauge (if the calibration is what I am thinking) should read around center.

BTW Mid-Fifty does have the gauge resistor. It's part #2688. I would call them and ask them about it. They also have a good gauge troubleshooting guide on their website. Go to the Instructions tab and drill down to Temperature Sender. They also give that part# there. If a resistor is what you need and they can not get you one, hopefully find a Radio-Shack that may still be in business. There are still a few still out there with left over parts that were a side business with locally owned Hardware Stores etc. I just went through this and that's where I found mine.